Salmonella spp. and Listeria monocytogenes shedding in domestic ruminants and characterization of potentially pathogenic strains.

Salmonella and Listeria monocytogenes are important human pathogens that can be isolated from the faeces of ruminants, and constitute potential sources of human infection via contamination of food and water or contact (direct or indirect) between humans and animals. A cross-sectional survey including 301 herds was carried out to estimate the prevalence of cattle herds (beef and dairy) and sheep flocks positive for these zoonotic organisms in ruminants in the Basque Country (Northern Spain). The proportion of herds where Salmonella shedding was detected was 5.8% for beef cattle, 3.7% for dairy cattle and 6.1% for sheep flocks. Apart from a multi-drug resistant (ASuT) monophasic variant of S. Typhimurium (4,[5],12:i:-) isolated from a beef cattle herd, the remaining 11 serotypes of S. enterica identified were not among the common causes of non-typhoidal salmonellosis in humans. L. monocytogenes was isolated from 42.3% of beef cattle herds, 46.3% of dairy cattle herds, and a significantly lower proportion of sheep flocks (23.5%). Presence of other animal species in the farm and geographical location were associated with herd risk of L. monocytogenes shedding. A total of 65.8% of the isolates of L. monocytogenes harboured the virulence gene, llsX, as determined by real-time PCR. In conclusion, ruminant herds in the Basque Country did not seem to pose a high risk for Salmonella serotypes commonly associated to human clinical cases of salmonellosis entering the food chain. Conversely, potentially pathogenic L. monocytogenes strains were widespread in ruminant farms, mostly in cattle.